Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
Description of the Related Art
In recent years, higher numbers of laser beams have been used in image forming apparatuses accompanying higher image formation speeds and higher image resolution in image forming apparatuses. Also, as electrophotographic image forming apparatuses (printers) continue to enter the light printing market, it is desired that high-quality image formation can be performed on various types of screens used in the printing market, even with an electrophotographic method.
However, in the case of increasing the number of beams used in an image forming apparatus, if small shifts in the position in the main scanning direction of pixels formed on a photosensitive member (photosensitive drum) by the light beams (laser beams) occur, such a shift will occur periodically in the sub-scanning direction. As a result, a moiré pattern appears due to interference with the screen. For example, with screens used in the printing market, it has been demonstrated that a moiré pattern (interference pattern) will appear if a position shift of 6 μm or more occurs between beams.
FIG. 16A is a diagram showing an example of a 50% half-tone screen image (hereinafter referred to as an “HT image”). The unevenness in density, which is caused by a shift in the image formation positions of multiple laser beams (phase shift) in the main scanning direction, appears as an interference pattern in the HT image shown in FIG. 16A. Note that the HT image shown in FIG. 16A is around 10 mm by 10 mm in area, and the interference pattern that is inclined 45 degrees to the left appears in cycles of about 1 mm. If there is no phase shift, this kind of interference pattern does not appear in the HT image, and a uniform halftone density appears on the HT screen. Also, FIG. 16B shows an enlarged image of a 50% ideal HT image, which is around 0.4 mm by 0.4 mm in area. A screen corresponding to a 50% HT area in which a pixel 1601 corresponds to one 1200-dpi pixel is shown.
Also, FIG. 17 shows an enlarged image of the 50% HT image shown in FIG. 16A, in which the density unevenness caused by the phase shift is enlarged. The thin lines shown in FIG. 17 are auxiliary lines that have been added to make the units of the screen easier to understand. FIGS. 16B and 17 show cases of scanning a photosensitive member using eight beams, in which a 0.5-pixel shift in the main scanning direction has occurred between two groups, namely laser beams 1 to 4 and laser beams 5 to 8, the phase shift between laser beam 1 and laser beam 8 is 0.5 pixels, and the image slopes by a ratio of 0.5 to 8 (corresponds to around 3.6 degrees). The periodicity of the phase shift can be checked using the difference in level in the auxiliary lines in FIG. 17.
In FIG. 17, the interference pattern influenced by the phase shift appears due to the differences in level in 8-beam periods, and interference at fine connection portions in the HT image. The HT fine connection portions correspond to portions that are opposed in an oblique arrangement in the alignment of pixels in the sub-scanning direction. Dotted line 1701 and dotted line 1703 in FIG. 17 show portions shifted in the direction in which the alignment of the pixels at the fine connection portions overlaps due to the difference in level at a border portion of the phase shift of the lasers. Also, dotted line 1702 shows portions shifted in the direction in which the overlapping of the fine connection portions is separated by the difference in level at the border portion of the phase shift.
The portions indicated by arrows on the dotted line 1701 and the dotted line 1703 are portions at which toner developing is more likely to be performed due to the influence of the fine connection portions. The portions indicated by arrows on the dotted line 1702 are portions at which toner developing is less likely to be performed due to the influence of the fine connection portions. These cause the interference pattern to appear. When the phase is shifted about 45 degrees in the left-oblique direction, the interference pattern appears in cycles of about 0.83 mm due to the interval between the dotted line 1701 and the dotted line 1703. Based on the structure of the fine connection portions, it is reasoned that the phase shift tends to increase monotonically until the phase is shifted by one pixel, or in other words, a ratio of 1 to 8 (about 7.1 degrees).
The reason that toner development is more likely to be performed at portions shifted in the direction in which the pixel alignment of the fine line connection portions overlaps is because the exposure distribution of one laser spot is not square-shaped, but forms a circular Gaussian distribution having a diameter of 1.5 to 2 pixels. Laser spots tend to exhibit a density that is higher than the number of pixels due to an increase in the overlapping of one pixel or less and exhibit a relatively lower density due to increases in the distance between spots of around one pixel or less, and therefore a regular interference pattern appears.
Conventionally, in order to deal with a phase shift such as that described above, the formation positions of pixels have been controlled by measuring the phase difference between beams with respect to the writing start positions of the pixels by the laser beams and adjusting the phases of the laser beams based on the measurement result (e.g., Japanese Patent Laid-Open No. 2008-89695). Furthermore, at multiple different positions in the main scanning direction, the amount of shifting between the formation position and the ideal position of a pixel is measured for each laser beam, and the partial magnification in each of multiple different regions in the main scanning direction is corrected based on the measurement result. Thus, a shift in the pixel formation positions from the ideal positions is prevented from occurring in the entire region in the main scanning direction scanned by the laser beams.
Also, in an image forming apparatus such as that described above, if the relative positions of the optical scanning apparatus that scans multiple laser beams and the photosensitive member (photosensitive drum) that is scanned by the multiple laser beams is not appropriate, the focus will shift when the laser beams scan the photosensitive drum. As a result, a phase shift in the laser beams such as that described above can occur. This kind of phase shift increases in amount the smaller the diameter of the photosensitive drum is and the larger the number of beams there are in the optical scanning apparatus. For example, if the diameter of the photosensitive drum is 30 mm and the number of beams is 16, there are cases where a phase shift of around 4 μm at most occurs between the laser beams on the two ends of the multiple laser beams, which are aligned in a straight line. For this kind of phase shift, a method is known in which the phase shift is corrected by outputting an image for measuring the phase shift and obtaining the phase shift from the output image.
Furthermore, in an image forming apparatus including multiple light emitting elements, if the internal temperature rises due to heat emitted from the light emitting elements and a polygon motor, optical characteristics (refractive index, etc.) of a scan lens and the like change and the relative scan positions of the multiple laser beams on the photosensitive drum change. That is to say, a position shift (phase shift) in the main scanning direction appears in the electrostatic latent image formed on the photosensitive drum by the laser beams. Accordingly, even if adjustment (correction) of the phase and correction of the partial magnification of the laser beams is performed as described above, the phase shift occurs due to temperature change in the image forming apparatus. Correction of a phase shift due to temperature change in the image forming apparatus (phase shift due to environmental variation) can be realized by, for example, obtaining the relationship (phase shift characteristic) between the temperature of the image forming apparatus and the phase shift that occurs in the multiple laser beams in advance using measurement or theoretical consideration.
However, in an image forming apparatus such as that described above, there is a possibility that the correction accuracy will deteriorate depending on the environmental conditions at the time of performing correction of a phase shift due to a shift in the relative positions of the optical scanning apparatus and the photosensitive drum (phase shift due to relative position shift). Specifically, since the environment at the time of acquiring correction data during factory assembly of the apparatus and the market usage environment are not the same, there is a possibility that a phase shift cannot be corrected with sufficient accuracy using the correction data stored at the factory. Also, in an image forming apparatus configured such that, in the market, an operator causes the apparatus to store correction data by using a test image output by the apparatus, the environmental state in which the test image is generated and the environmental state at the time of subsequently forming an image are different, and therefore there is a possibility that a phase shift cannot be corrected with sufficient accuracy using the correction data generated from the test image.